1. Field of the Invention
The present invention relates to a self-priming centrifugal pump particularly of the kind having built-in ejector.
Self-priming pumps of this type, generally termed "jet pumps", comprise, inside the pump casing, an ejector which is connected to the intake port on one side and to the inlet of the impeller on the other.
2. Description of the Related Art
As is known, the impeller of said pumps must generate a total flow Q which is expressed by the formula: EQU Q=Q.sub.1 +Q.sub.2
where Q.sub.1 indicates the useful flow delivered by the pump and Q.sub.2 indicates the partial flow which flows through the ejection nozzle. The flow Q.sub.2, on the basis of the known operating principles of ejectors, draws into to the ejector's negative-pressure chamber a flow Q.sub.1 which arrives from the intake port. Said flow Q.sub.1 mixes in the diffusion duct of the ejector with the flow Q.sub.2 and is then conveyed toward the inlet of the impeller to be subsequently recirculated within the case.
The method of operation of said self-priming pumps is as follows. Initially, the case of the pump must be entirely filled with liquid up to the intake port which is located above the longitudinal axis of the impeller. In this manner the ejector is also completely filled with liquid to be pumped.
When the pump is started, the impeller imparts a vorticose motion to the liquid, forming a mixture of air and liquid which is discharged into the upper portion of the case, where the separation of the air can occur at low speeds. The separated air partially flows to the delivery port and is partly entrained within the liquid flowing toward the ejection nozzle, where it gradually draws more liquid toward the inlet of the impeller. The recirculation of the air/liquid mixture continues until all the air is eliminated, after which the normal operation of the pump can begin.
By means of such a pump-ejector combination it is possible to automatically prime the system, lifting fluids even from considerable depths, up to approximately 9 meters and above. Said devices, however, are not free from disadvantages, including most of all long priming times and low efficiency during normal running conditions.
It has been experimentally demonstrated that the longer priming times correspond to conditions of greater turbulence of the air/liquid mixture which leaves the impeller. Said priming times are also further increased if the flow of the air/liquid mixture is proximate to the delivery port, so as to prevent the separation of air from the mixture and reduce the efficiency of the ejector. Therefore, in order to reduce priming times and increase the overall efficiency of the pump, it is necessary to carefully study the conditions of outflow at the outlet of the impeller and its re-conveyance toward the ejector.
In order to obviate this disadvantage, a self-priming ejector pump has been provided in which the flow leaving the impeller, initially guided by an annular diffuser, is subsequently conveyed toward an essentially frustum-shaped interspace and finally discharged through an arcuate slot which faces the intake port of the pump. Inside said frustum-shaped interspace there is a deflector blade which is connected to one of the front chambers of the annular diffuser. The priming times of said pump are considerably reduced down to 5-6 minutes; however, the efficiency of the pump-ejector assembly during normal running conditions is still not adequate. This is due to the fact that the outflow of the mixture through the arcuate slot is still predominantly turbulent and does not ensure a uniform feeding of the ejection nozzle.